Processes of coating



June 1957. w. T. JOHNS 2,795,522

' PROCESSES 0F comma Fi-led Aug. 19, 1955 FILM OR'PAPER WEB INVENTOR WALTER T. JOHNS ATTORNEY PROCESSES F COATING Walter '1. Johns, Lynbrook, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application August 19, 1953, Serial No. 375,127

9 Claims. (Cl. 154-102) This invention pertains to processes of coating viscous aqueous solutions. More particularly, it pertains to .a process of coating viscous aqueous solutions as layers in photographic films and papers. Still more particularly it pertains to a process of coating viscous waterpermeable colloid silver halide dispersions onto continuous webs.

Various processes of coating viscous aqueous solutions and particularly aqueous photographic emulsions have been proposed. Among these processes are those wherein the coating and drying chambers are at a reduced pressure. However, in such continuous coating operations a substantial amount of air bleeds into the system as the film or paper passes into the coating cham-.

her and this among other factors has placed limitations on the coating speed. In the prior art procedures coating speeds up to about 120 feet per minute have been obtained.

An object of this invention is to provide a process of coating, viscous aqueous solutions onto continuous webs at rates of speeds much faster than 120 feet per minute.

Another object is to provide such a method wherein the coatings are of uniform thickness and free from entrapped bubbles between the coating and the web. A further object is to provide such a method wherein extremely thin coatings can be made. A still further object is to provide a method of coating water-permeable colloid dispersions and especially colloid silver halide dispersions onto continuous film or papers at high rates of speed. Yet another object is to provide such a method which can be carried out in a relatively simple apparatus. Still other objects will be apparent from the following description of the invention.

The above objects are obtained in accordance with the present invention which, in its broadest aspects, comprises extruding an aqueous solution of high viscosity through a narrow orifice onto a moving web traveling at a higher linear speed than the fluid being extruded, in the substantial absence of non-condensable gases, but in the presence of a condensable vapor of an inert, water-v soluble liquid having a boiling point at atmospheric pressure not greater than about 212 F. at an absolute pressure from 20 to 190 mm. of mercury and at a temperature from 75 F. to 150 F.

In a preferred aspect of the invention, a viscous aqueous dispersion of light-sensitive silver halides in a waterpermeable, natural or synthetic colloid is extruded in the form of a thin sheet onto a continuous flexible web, e. g., a thin film base or paper that is traveling at a linear speed of at least 150 feet per minute and from 1 to 200 times faster than the rate of extrusion of said dispersion, while adding to the coating zone the condensable vapor of a photographically-inert, water-soluble liquid having a boiling point at atmospheric pressure not greater than about 212 F., at an absolute pressure from 20 to 190 Patent 0 M mm. of mercury, the partial pressure of the non-condensable gases in said zone being not more than 5% of the 2,795,522 Patented June 11, 1957 total pressure, and maintaining said zone at a temperature from 75 F. to 150 F.

The total pressure in the coating zone, by reason of the addition of the vapor, should be sufficient to prevent the water-soluble solvents (including Water) in the aqueous solution being coated from boiling at the coating temperature (e. g., 75 F. to 150 F.). The partial pressure of the non-condensable gases, mainly oxygen and nitrogen, being maintained at less than 5% of the total pressure means that the vapors remaining in the coating zone are substantially all condensable. complished in various ways, e. g., by first reducing the pressure in the system to below the operating pressure, for instance, 2 to 3 mm. of mercury and adding the condensable vapor of the water-soluble liquid to the coating zone to raise the total pressure to 20 to 190 mm. of mercury, or by purging the zone with condensable vapors at a higher pressure.

Suitable condensable vapors include the vapors of such inert, water-soluble liquids as water, methyl alcohol, ethyl alcohol, and acetone. These condensable vapors are introduced into the system at a rate in excess of the rate of evaporation or removal of water and water-soluble solvents from the coated water-permeable colloid layer carried by the film. The minimum pressure of condensable vapors in the reduced pressure coating zone is equal to or slightly greater than the vapor pressure corresponding to the boiling point of the Water and/or solvents in the coated layer at the reduced pressure. The water or solvents in the coated layer are removed, to the desired extent, at a later stage, for instance, by a drying operation.

It has been found that by supplying Water vapor to a coating zone maintained at a pressure of less than 190 mm. of mercury and keeping the partial pressure of noncondensable gases in the system at less than 5% of the total pressure while coating an aqueous dispersion of silver halides in a water-permeable colloid having a viscosity of 2,000 to 100,000 centipoises, and from 20 to total solids content, coating speeds greater than 150 feet per minute can be obtained. Thin coatings of the order of 0.05 to 2.5 mils in thickness which are free from entrapped bubbles and have uniform adherence to the base can be obtained from extruded aqueous films 2 to 30 mils in thickness at temperatures from F. to F. This is surprising in view of the fact that when non-condensable gases, e. g., air are present in an amount as low as 10% of the total vapor pressure, non-condensable vapors are entrapped between the extruded sheet and the flexible base at an increased rate as coating speeds are increased. The fact that the addition of water vapor to the system results in uniform adherence and markedly reduces bubble entrapment, even at five-fold increases in speed over the prior art methods, is not obvious from prior art.

The novel combination of greatly reduced pressure and the positive addition of a condensable vapor, e. g., water vapor to the coating zone in the processes of this inven tion apparently enables the manifold increases in speed i to be obtained because the mere addition of water vapor to the coating zone at atmospheric pressure or reduced pressures down to 5 inches of mercury absolute do not permit more than about a 10% increase in coating speed without deleterious bubble entrapment. p

The invention is useful in the coating of aqueous silver halide dispersions in the various types of natural and synthetic water-permeable colloids which have been used as binding agents for the silver halide grains, including gelatin, albumin, agar-agar, water-permeable polyamides, polyvinyl alcohol, partially hydrolyzed polyvinyl esters,

hydrophilic polyvinyl acetals including those containing color-former n'ucleij and other types of viscous aqueous This can be ac-- solutions, particularly those containing colloids. It is particularly useful in the coating of gelatinosilver halide emulsions.

The invention will be further illustrated but is not intended to be limited by the followingexarnples.

Example I At the beginning of a coating operation a coating zone is reduced to a pressure of about 2 to mm. of mercury to remove substantially all non-condensable gases (i. e., air), water vapor is then added raising the pressure to 50 to 60 mm. of mercury, while continuing the source of vacuum. A gelatino silver bromide emulsion, containing 50% by Weight of solids (including silver halides) and having a viscosity of about 10,000centipoises, is then extruded through a hopper provided at' its lower end with lipsv spaced approximately 0.005 inch apart,'at a rate of about 80 feet per minute'onto a cellulose acetate film base having a thickness of approximately 0.003 inch and bearing a thin gelatin sub-coating on its upper surface, which is moving through the coating zone at a rate of approximately 400 feet per minute. The coating zone is maintained at a reduced pressure of about 50 to 60 mm. of mercury and a temperature of about 100 to 107 F, while water vapor is added at a rate. exceeding the rate of evaporation of water from the coated aqueous gelatin silver halide emulsion. The gelatin silver halide coating, as it passes from the reduced pressure zone, is free from entrapped bubbles, waves and streaks, is uniform in character and sufiiciently firm so that it can be further dried and cured at ordinary or raised temperatures, and the coated film has satisfactory photographic properties.

Example II A continuous sheet of paper bearing a baryta coating is passed into a coating chamber which is evacuated to a reduced pressure of about 6 mm. of mercury and maintained at a temperature of about 100 F. to 107 F. Water vapor is passed into the chamber while substantially all of the air is removed from the chamber and the pressure raised to'about 60 mm. of mercury. An aqueous gelatino silver bromide photographic emulsion having a solids content of about 40% and a viscosity of 7,000 centipoises is coated in the absence of actinic light onto the baryta surface of the paper through a narrow slot orifice, the lips ofwhich are spaced about 0.01 inch apart, extruding at a rate of 30 feet per minute, said paper web being moved at a rate of 300 feet: per minute, while continuously adding water vapor and maintaining the pressure at about 50 mm. of mercury'and the temperature at about 100 F. The photographic paper sheet, as it passes from the reduced pressure coating zone, has satisfactory photographic properties, is smooth, free from entrapped bubbles and at normal pressure is sufficiently firm so that it can be further dried at normal or elevated temperature then passed to a wiirdup stationor slit and cut into small sheets.

Example III A continuous length of cellulose acetate film base supporting a gelatin substratum is fed into a coating chamber. The pressure is reduced to about 50 mm. Hg absolute and the temperature maintained at about 100 F. Water vapor is admitted to the chamber while continuing evacuation to maintain a constant pressure and purge the chamber of non-condensable gases. An aqueous gelatin solution of about 40% total solids, containing a water-soluble anti-halation dye and having a viscosity of 30,000 centipoises is then extruded through a hopper having at the bottom a narrow slot orifice wherein the lips are spaced about 0.020 inch apart, at a rate of approximately 40 feet per minute, onto thefilm base and gelatin sub moving at a rate of- 400 feet per EDescribed in Example I of GasparU. S. Patent. 2,214,782.

"free'from entrapped bubbles and streaks, is uniform in character'and has satisfactory photographic properties.

Various types of coating apparatus may be used in carrying out the process. The essential point is to surround the coating zone with a suitable air-tight chamber provided with means for removing air and vapor from the chamber and with means for introducing the readily condensable vapor of a photographically-inert, watersolubleliquid. A suitable apparatus is shown schematically in the accompanying. drawing wherein 1 is a chamber into which a web of paper or filmpasses over a guide roller 2 and thence past pinch rolls 3 which press it against a revoluble sealing drum 4 under guide rollers 5 and 6, then upwardly and over a reller 7 which constitutes the point of coating, thence, downwardly under guide rollers 8 and 9, then upwardly through a constricted passage between .the sealing drum 4 and the curved wall 10 of the chamber 1 and then to a drying section and a windup reel (not shown). The chamber is provided with openings 11 which areconnected by means of a suitable pipe 12 to .a vacuumpump or aspirator (not shown). A pipe 13 also passes through the wall of the chamber and is provided with a suitable nozzle 14 for introducing a eon-densable vapor of a liquid, e. g, water into the chamber. A pipe 15 communicates with a coating bar 16 which is provided with a longitudinal chamber 17 and a narrow slot orificej18 is connected with a suitable metered source of a viscous aqueous dispersion of silver halides in gelatin. .The temperature of the chamber can be raised by means of heating elements 19 or a Water jacket provided with suitable sources of power. The wall of chamber 1 opposite the point of coating is provided with a window'20 so that the coating operation can be observed.

In operation, the chamber 1 is connected with a vacuum pump and the pressure in the chamber is reduced to about2 to 3 mm. of Hg absolute. Water vapor is then introduced through pipe 13 raising the pressure to about 60 and the web to be coated is moved past the coating bar 16 while the coating material is extruded onto the Web which moves at a speed greater than the speed of extrusion so that the extruded 'film is drawn down. The vacuum is maintained during the coating operation and water vapor is continuously passed into the chamber through the pipe 13.

The invention is, of course, not limited to the coating of'filfu and paper with aqueous gelatin silver halide emulsionsbut is useful in coating aqueous silver halide dispe'rs'ions in other water-permeable colloids as described above. Thus, it can be used in the coating of aqueous ethanol solutions of polyvinyl acetal color formers of the type described in Jennings et al. U. S. Patent 2,397,864 and Blanchard U. S. Patent 2,5 5 1,091.

The invention is useful in preparing multilayer photographic film elements including multicolor films both positive and negative, printing papers, double-coated X-ray films, motion picture films, portrait films, and papers, etc. It is not only useful in coating the lightsensitive silver halide colloid layers but can be used in coating light-filter layers, and anti-halation layers containing dyes and pigments which absorb the desired wavelengths of light, as well as antiabrasion layers and sublayers from aqueous solutions, including aqueous ethanol solutions. 'In these layers gelatin or the other colloids described above may constitute the film-forming binding agent. The coating solutions may have a solids content of 25% to 60%.

An advantage of the'invention is that it provides a process ofcoating aqueous colloid layers at high rates ofspe'ed. It is particularly useful for viscosities of 30,000 to 60,000 centipoises. The process is simple to operate and: operates successfully'ona continuous basis. Another advantageis .thatthe process enables one to use silver halide emulsions of high solids content with a resultant marked decrease in drying time. Yet another advantage is that the process admits of more uniform weight control of the silver halide coatings independent of emulsion viscosity, by positive metering of the emulsion through the coating orifice. A further advantage is that the process eliminates the need for chilling the aqueous gelatin silver halide emulsion coating in order to set it for further drying treatment. The manifold increase in coating speed makes the process more economical than prior art processes and produces coatings of satisfactory quality.

What is claimed is:

1. The process which comprises extruding a viscous aqueous solution in the form of a sheet onto a moving flexible web traveling at a linear speed of at least 150 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the solution, in the substantial absence of non-condensable gases, but in the presence of added readily condensable vapors of an inert, watersoluble liquid taken from the group consisting of water, methyl alcohol, ethyl alcohol and acetone while maintaining a pressure from 20 mm. to 190 mm. of mercury and a temperature of 75 F. to 150 F.

2. The process which comprises extruding a viscous aqueous solution containing a water-permeable colloid in the form of a sheet onto a moving flexible web traveling at a linear speed of at least 150 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the solution, in the substantial absence of non-condensable gases, but in the presence of added readily condensable vapors of an inert, water-soluble liquid taken from the group consisting of water, methyl alcohol, ethyl alcohol and acetone while maintaining a pressure from 20 mm. to 190 mm. of mercury and a temperature of 75 F. to 150 F.

3. The process which comprises extruding a viscous aqueous dispersion of silver halides in a water-permeable colloid in the form of a sheet onto a moving flexible web traveling at a linear speed of at least 150 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the dispersion, in the substantial absence of non-condensable gases, but in the presence of added readily condensable vapors of an inert water-soluble liquid taken from the group consisting of water, methyl alcohol, ethyl alcohol and acetone while maintaining a pressure from 20 mm. to 190 mm. of mercury and a temperature of 75 F. to 150 F.

4. The process which comprises extruding a viscous aqueous dispersion of silver halides in a water-permeable colloid through a narrow slot orifice in the form of a sheet onto a moving flexible web traveling at a linear speed of at least 150 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the dispersion, in the substantial absence of non-condensable gases, but in the presence of added readily condensable vapors of an inert, water-soluble liquid taken from the group consist ing of water, methyl alcohol, ethyl alcohol and acetone while maintaining a pressure from 20 mm. to 190 mm. of mercury and a temperature of 75 F. to 150 F.

. 5. The process which comprises extrudlng a viscous aqueous dispersion of silver halides in a water-permeable colloid having a viscosity of 2,000 to 100,000 centipoises through a narrow slot orifice from .002 to .030 inch in thickness in the form of a sheet onto a thin flexible, moving web traveling at a linear speed of 350 to 500 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the dispersion, in the substantial absence of air, in the presence of added readily condensable vapors of an inert water-soluble liquid taken from the group consisting of water, methyl alcohol, ethyl alcohol and acetone while maintaining a pressure from 20 mm. to 190 mm. of mercury and a temperature of F. to 150 F.

6. The process which comprises extruding a viscous aqueous dispersion of silver halides in a water-permeable colloid having a viscosity of 2,000 to 100,000 centipoises through a narrow slot orifice from .002 to .030 inch in thickness in the form of a sheet onto a thin flexible, moving web traveling at a linear speed of 350 to 500 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the dispersion, in the substantial absence of air, in the presence of added water vapor while maintaining a pressure from 20 mm. to 190 mm. of mercury and a temperature of 75 F. to 150 F.

7. The process which comprises extruding an aqueous dispersion of light-sensitive silver halides in gelatin having a viscosity of 3,000 to 60,000 centipoises, in the form of a thin sheet from 0.002 to 0.030 inch in thickness, onto a thin flexible moving web traveling at a linear speed of,

350 to 500 feet per minute and 1 to 200 times faster than the linear speed of extrusion of the dispersion, in the substantial absence of air and in the presence of added water vapor while maintaining a pressure from 30 mm. to mm. of mercury and a temperature of 85 to F., said air being present in an amount less than 5% of the total vapor pressure and the water vapor being added in an amount sufiicient to keep the water in the aqueous dispersion from boiling at said coating temperature.

8. A process of coating antihalation layers which comprises extruding a viscous aqueous solution containing a water-permeable colloid and an antihalation dye and .having a viscosity of 2,000 to 100,000 centipoises in the form of a sheet onto the surface of a thin flexible film base which is moving at a linear speed of at least feet per minute and 1 to 200 times faster than the linear speed of extrusion of said solution, in the substantial absence of air, in the presence of added water vapor while maintaining a pressure from 20 mm. to mm. of mercury and a temperature of 75 F. to 150 F.

9. A process as set forth in claim 8 wherein said colloid is gelatin.

References Cited in the file of this patent UNITED STATES PATENTS 1,164,174 Devine Dec. 14, 1915 2,175,125 Mack et a1. Oct. 3, 1939 2,681,294 Beguin June 15, 1954 FOREIGN PATENTS 367,386 Great Britain Feb. 12, 1932 448,863 Great Britain June 10, 1936 

1. THE PROCESS WHICH COMPRISES EXTRUDING A VISCOUS AQUEOUS SOLUTION IN THE FORM OF A SHEET ONTO A MOVING FLEXIBLE WEB TRAVELING AT A LINEAR SPEED OF AT LEAST 150 FEET PER MINUTE AND 1 TO 200 TIMES FASTER THAN THE LINEAR SPEED OF EXTRUSION OF THE SOLUTION, IN THE SUBSTANTIAL ABSENCE OF NON-CONDENSABLE GASES, BUT IN THE PRESENCE OF ADDED READILY CONDENSABLE VAPORS OF AN INERT, WATER- 